I can't see that little line of colour. What XP does with tabs, by making the active tab flat and the rest gradients, is exactly why I can see it.

Give me contrast by using a mix of gradients and flat, any day.

Presuming the active tab is flat white either way; what's the difference in legibility between the inactive tabs being gradiented white-to-grey (the XP Windows controls style) vs. inactive tabs just being flat grey (the "modern UI" e.g. Google Chrome style)?

I would note that Windows XP also had a high-contrast theme; and that, when enabled, inactive tabs actually lose the distinction in background color from active tabs. But they keep the highlight stripe.

Gradients are higher contrast than flat colours.

To try and put it another way, imagine, for a moment, that you are incapable of perceiving edges. Every single one of your flat tabs now looks like the same part of the same blob.

Now, realise that for a pretty high number of low-vision people, that is reality. The slight blur we experience makes most edges disappear.

(WinXP's high contrast overcame this by tripling the size of all edges).

Wouldn't the inactive flat tabs all still end up looking like one continuous blob either way? The gradient on the inactive tabs in the image I linked is a top-to-bottom gradient; it does nothing to enhance the visibility of the edges between contiguous inactive tabs.

However, the most important piece of information, the _active tab_ is easily discernible.

Switching tab is easy enough, by clicking the centre of its label. When you wish to switch tab, you generally are going to read the label.

But when working out where you are, that can be total guesswork.